The present invention is directed to an apparatus for generating an analog output signal representative of a digital input signal, which digital input signal is received by the apparatus as a plurality of bits conveyed by a plurality of bit paths. Parameter generating apparatuses, such as current mirrors, are often employed to provide a predetermined parameter to various portions of a device. Specifically, in digital-to-analog conversion apparatuses, current mirrors are often employed to generate a reference current and a plurality of output currents or characteristic currents in respective output current branches for providing analog signals responsive to respective digital bits comprising the received digital input signal.
In newer circuit designs, a lower supply voltage, on the order of 3.0 volts or less, is employed. As a consequence, the digital-to-analog converter design techniques previously relied upon (which employed a higher supply voltage--on the order of 5.0 volts) can no longer be used.
Current mirrors are generally not temperature stable. That is, the respective output or characteristic currents do not track with the reference currents as temperature changes, and the device is sensitive to a mismatch of threshold or trigger voltages of the various transistors employed therein for establishing the reference current and the output or characteristic currents.
One prior art solution to such temperature-related instability was to "stack" multiple current mirror devices, generally two high, in a manner such that a switching transistor in the first output stage effectively shields the switching transistor in the second output stage so that the source of the first stage transistor and the drain of the second stage transistor tracked together, thereby generating a temperature-stable output current in the second stage output transistor. However, the newer digital-to-analog converter designs which employ a lower supply voltage are not amenable to such a "stacked" device since the lower power supplied does not accommodate such multiple devices.
There is, therefore, a need for a stable, more accurate digital-to-analog converter apparatus which can accommodate a lower supply voltage.